1. Field of the Invention
The present invention relates to lithography, and more particularly, to projection optics used in lithography.
2. Background Art
Lithography systems are used to print features in a variety of manufacturing applications. Photolithography systems use a mask or reticle to expose features onto an object. In semiconductor manufacturing, for example, a reticle is exposed by an exposure beam. An optical system then projects a reduced image of the reticle onto a silicon wafer. In this way, circuit features can be printed on a semiconductor substrate.
Maskless lithography systems have been developed that do not require use of a mask or reticle. Current maskless lithography systems project a pattern to be printed onto a moving object. For example, a pattern of circuit features can be projected onto a moving wafer or flat panel display. In one example, a silicon wafer can be coated with a photoresist. The pattern is projected on the wafer using one or more pattern generating devices, such as spatial light modulator (SLM) arrays. This SLM array is a programmable array of elements that modulates the light projected onto the object. Types of SLMs can include, for example, digital micromirror devices (DMD), transmissive liquid crystal light valves (LCLV), and grating light valves (GLV).
Projection optical systems are used to transmit light generated by a pattern generator, such as an SLM or a static light source used in lithography systems that use masks, to a wafer to create an image on the wafer. Existing projection systems have significant limitations. Among these limitations are that existing systems often include a very large number of optical components and are often complicated. For example, in one case the projection optical system includes three relays. The first relay is an on-axis module, while the other two are off-axis modules. Each module must be corrected independently. Aberrations of on-axis and off-axis modules are not compensable. Furthermore, the alignment process is complicated and overall dimensions are large. Other limitations are found in systems that limit light transmission to only about 25% of the light provided by the pattern generating device.
What is needed is a projection system that provides high performance with reduced complexity.